1. Field of the Invention
this invention relates to magnesium alloys having improved corrosion resistance and, in particular, to an improved form of the alloy known commercially as AZ91, being nominally 9% Al, 1% Zn, 0.15% Mn with the balance magnesium.
2. Discussion of the Background and Description of Related Art
The ASTM specification limits for other elements appearing as impurities in the highest purity AZ9D magnesium alloy are: Fe, 0.004%; Ni, 0.001% and Cu, 0.015%. The improvement in corrosion resistance which results from maintaining the concentration of these heavy metal elements at a low level was described in U.S. Pat. No. 2,264,309 issued Dec. 2, 1941 to Hanawait et al. Hanawait points out that a "pure" alloy may have a corrosion resistance at least equal to that of magnesium alone. He goes on to state: "Such a `pure` alloy, however, is not as workable in all aspects as the commercial alloy and further it is improbable that it could be made generally available economically" (page 1, 1st column lines 50 to 54). As discussed at pages 670-685 of "Principles of Magnesium Technology" by Emley (Pergamon Press, 1966) the existence of a "tolerance limit" was noted and when an element, typically Fe, is present in excess of this limit the corrosion rate rises rapidly. It was further noted in SAE Technical Papers Nos. 830523 and 860288 (International Congresses, Detroit, Mich., Feb. 28, 1983 and Feb. 24, 1986) that the tolerance limit for Fe is affected by the amount of the major alloying elements and, specifically, varies directly with the amount of manganese in the alloy. These tolerance limits have been incorporated into the impurity specification limits for AZ91D. The typical composition of AZ91B given in paper 830523 is a magnesium base with 8.5 to 9.5% aluminum and 0.45 to 0.9% Zn.
These publications indicate that below the tolerance limit the corrosion rate is essentially constant. Unexpectedly, it has now been found that this is not accurate and that the corrosion rate decreases in a logarithmic relationship with decreasing impurities. Based on this discovery two new alloys of differing composition, both with low concentration of heavy metal impurities, have been prepared and found to have desirably low corrosion rates.